This invention relates to a decision-feedback equalizer for use in equalizing a demodulated signal sent from a demodulator.
In general, a demodulator is operable in response to a received signal sent through a radio channel to demodulate the received signal into a demodulated signal. The received signal may have been seriously subjected to distortion during transmission. The demodulated signal may be a baseband signal having a binary level or multilevel. A decision-feedback equalizer of the type described is supplied with the demodulated signal and is operated to equalize the demodulated signal into an equalized signal. As a result, the distortion is removed from the equalized signal which is put into a distortion-free state.
A conventional decision-feedback equalizer comprises first and second transversal filters each of which has a plurality of controllable taps and which are supplied with the demodulated signal and an input signal which is produced on the basis of the equalized signal in a manner to be described later. The first transversal filter filters the demodulated signal into a first filtered signal in accordance with a plurality of main controllable tap gains of the respective taps. On the other hand, the second transversal filter filters the input signal into a second filtered signal in accordance with a plurality of subsidiary controllable tap gains. The conventional decision-feedback equalizer further comprises a producing section for producing the equalized signal dependent upon the first and the second filtered signals and a control section for controlling the main and the subsidiary controllable tap gains in accordance with the demodulated and the equalized signals.
Such a conventional decision-feedback equalizer is disclosed in "DIGITAL COMMUNICATIONS" which is issued by McGRAW-HILL INTERNATIONAL BOOK COMPANY and which is written by John G. Proakis, Professor of Electrical Engineering in Northeastern University.
However, the distortion inevitably and partially remains as a residual distortion in the equalized signal in the above-referred decision-feedback equalizer when the received signal is subjected to a large distortion through the radio channel. As a result, the main and the subsidiary controllable tap gains are not quickly controlled by the control section because the residual distortion remains in the equalized signal. Therefore, it is difficult for the conventional decision-feedback equalizer of the above-mentioned type to quickly equalize the demodulated signal into the equalized signal and to obtain a distortion-free equalized signal.